Electronic computing devices have become increasingly important to data computation, analysis and storage in our modern society. Modern direct access storage devices (DASDs), such as hard disk drives (HDDs), are heavily relied on to store mass quantities of data for purposes of future retrieval. As such long term data storage has become increasingly popular, and as the speed of microprocessors has steadily increased over time, the need for HDDs with greater storage capacity to store the increased amount of data has also steadily increased.
Consequently, there are seemingly constant development efforts to improve the areal density of the media implemented in hard disk drives, where the areal density is typically measured as the product of bits per inch (“BPI) and tracks per inch (“TPI”). BPI refers to the number of bits that can be written and later reread per linear inch along a track, whereas TPI refers to the number of individual tracks per radial inch.
HDDs are configured with read/write heads, supported on air bearing sliders (ABS), for reading data from and writing data to rotating disks. One distinguishing characteristic of hard disk technology that makes it different from how floppy disks, VCRs and tape decks operate, is that the read/write heads are typically not designed to make contact with the media during read and write operations. Essentially, the reason for this is that due to the high speed at which the disks spin, and the need for the heads to frequently scan from side to side to different tracks, allowing the heads to contact the disk would result in unacceptable wear to both the delicate heads and the media.
Fly Height
A typical air bearing slider floats over the surface of the disk during read and write operations such that the head does not physically touch the corresponding disk. The amount of space between a head and a corresponding disk is called the “flying height” or “fly height”. The slider assemblies are spring-loaded, using the spring characteristic of the corresponding suspension or arm (i.e., a suspension preload), which causes the slider on which the head is coupled to press against the disk when the disk is stationary. When the disk spins up to operating speed, the high speed causes air to flow under the sliders and lift them off the surfaces of the disk. The spring load of a given slider is characterized by the “gramload” corresponding to the slider.
The term “gramload” generally refers to the spring load of the entire suspension and slider assembly but nevertheless may be referred to as “head gramload” or simply “gramload.” Gramloads are a characteristic of an HDD that is typically adjustable during manufacturing, e.g., to compensate for fly height deviations caused by slider crown, camber, etch depths, tolerances, etc. However, in order to accurately adjust the gramloads to compensate for fly height design deviations, one has to accurately measure the gramload of each head. Instruments exist for measuring gramload, however there are issues with their accuracy.